rails/rails — pull request #3479 by arvida (2011-10-31 11:13:24+00:00)
Title: Ensure Date header on expires_in

**Background:** Some webservers (thin and passenger for example) don't add a HTTP Date header to the response. This is not a problem for modern browsers but it can cause problems with other HTTP clients, for example Amazon's CloudFront [expects a Date header](https://forums.aws.amazon.com/thread.jspa?messageID=284053) when it gets a Cache-Control max-age value and .Net's HttpWebRequest's have [some similar issue](http://stackoverflow.com/questions/4328439/httpwebrequest-with-caching-enabled-throws-exceptions).
From what I understand the HTTP spec says that the client should use the Date header when calculating how a long a cached response is valid.

I can see that the Date header might not be Rails responsibility, but I think it would be good if ``#expires_in`` ensures that the client can use Cache-Control's max-age value by setting a Date header in the response. What you think about this?